This invention relates generally to optical storage. Particularly, this invention relates to a system providing enhanced astigmatic focus signal detection using a confocal aperture.
In October 2000, a study released out of the University of California, Berkeley reported that it took the entire history of humankind until the year 1999 to accumulate 12 exabytes of information (where 1 exabyte equals 1,000,000,000,000,000,000 bytes), which is 50,000 times the size of the Library of Congress. The study estimates that this amount of accumulated information will double by the middle of the year 2002. The incredible recent growth in the production of information has made the demand for information management tremendous.
Over the course of computing history, various methods of information storage have been used. Currently, optical storage is particularly popular, both because it is a removable storage means (in contrast to hard disk drives) and because it offers high capacity at a reasonable cost. Of course, as with many industries, competition is focused on three factors: bigger, faster, and cheaper. With respect to“bigger,” optical disk manufacturers strive to provide higher capacity disks. Currently, an everyday CD-ROM or CD-RW can store about 600 megabytes of information while a single-sided DVD traditionally can store 4,700 megabytes (i.e., 4.7 GB). However, the recent arrival of dual-layer optical technology was a jump for disk capacity. By writing and reading data encoded on two different depths or layers of the disk, capacity essentially doubled. For example, a dual-layer DVD can store 8,500 megabytes (i.e., 8.5 GB) per side. U.S. Pat. No. 5,993,930 to Hector et al. teaches one approach to a dual-layer optical medium having a multi-layered spacer layer between reflecting layers.
Multi-layer optical systems have new problems not necessarily present in the standard single-layer systems. For example, in a single layer system, the access head must be focused, and remain focused, on the sole information-carrying layer. Unfortunately, optical disks are not perfectly flat. As the disk spins during operation, the disk can tilt. These and other aberrations result in the information layer varying in distance from the access head and thus causing a need for the optical system to perform auto-focusing corrections. There are a handful of standard approaches to providing auto-focusing functionality to an optical storage system, including the astigmatic, Foucault, critical angle method, and differential methods. Of this group, the astigmatic method is perhaps most widely used.
In an astigmatic auto-focusing system, the objective lens of the access head is connected to a feedback mechanism which keeps the access head in focus with respect to the spinning and imperfectly flat optical disk. The signal which controls the feedback mechanism is often created by a photodiode array. Between the optical disk and the photo array is placed an astigmatic lens. As reflected light from the disk passes through this astigmatic lens, a system which is out of focus will create a light spot which is elliptical on the photo array. By sensing which direction the light is elliptically elongated, the photo array creates the appropriate signal to move the access head further from or closer to the optical disk, thus bringing it into focus.
In systems having more than one information-carrying layer, it is necessary not only to maintain focus on the current information layer, but to configure the access head to be able to selectively focus on any of the layers. For example, in a dual layer disk, the head will need to focus sometimes on the first layer and sometimes on the second layer. Astigmatic focusing has been adopted to assist with this need. For example, U.S. Pat. No. 5,811,789 to Nix provides an invention which selectively focuses a head on the desired data layer of an multi-layer optical medium. In the Nix invention, an astigmatic cylindrical lens is placed in the path of light between the data bearing surface and the access head's photodiode array. The photodiode array generates a focus error signal (“FES”) which is used by a servo assembly to bring the lens into focus on the appropriate information layer.
However, astigmatic focus is more difficult to accomplish on multi-layer optical disks than on single layer disks. In a disk have two or more layers, the information-storing layers of medium are closely spaced with all but the furthest information layer semi-transparent rather than entirely reflective. In a single layer system, all of the reflected light is caused by the reflection from that single layer. In a multi-layer system, when accessing the top information layer, only a portion of the reflected light is caused by a reflection from the top layer. Some light passes through the top layer to the second (and subsequent layers). These subsequent layers also reflect a portion of the light back to the astigmatic lens. These additional sources of reflected light keep the read-and-focus photo array detector from operating properly, rendering the focus error signal unreliable.
One solution is to use thick spacer layers between disk information layers. This introduces spherical aberration resulting in a larger spot and therefore in the end reducing the allowable information density. Another solution is to use an astigmatic detector lens that creates a “steep” FES S-Curve that goes through its peak and valley for less defocus motion. This results in a small spot at the quad detector, making the system more sensitive to lateral drifts.
What is needed is a system for a multi-layer optical storage unit which offers improved focus signal. Such a system should be economical and easy to implement without causing a degradation in spot quality at the disk or in the introduction of complicated assemblies. Ideally, what is needed is a way to remove the extraneous reflected light so that the photo array detector can function properly.